@OP - I'm guessing this is a question about residential, but I might have missed that. In addition to the other good advice in this thread, I would offer the following:
Adjusting for any kind of non-standard financing starts with the confirmation interview. If the sale price was negotiated first and non-standard financing done later, then there is usually no effect on the sale price. Once the sale price is negotiated, it generally doesn't matter where the funds to close come from, unless the sale price was re-negotiated based on the financing.
Think of it this way: you as the buyer will negotiate the sale price based on how you expect to finance the purchase (all debt, all equity, or some combination). If the buyer expected to pay cash or obtain 3rd-party financing and negotiated the purchase price based on that, then the non-standard financing has no effect. Attempts to derive a financing adjustment using cash equivalency or paired sales are a paper exercise, in that example.
OTOH, if the buyer and seller started negotiating and one of them mentioned mortgage assumption or owner-financing, that may color the sale price negotiations. Seller may take a lower purchase price to get an income stream. Buyer may pay above market to avoid mortgage underwriting and the potential of a lower equity requirement in non-standard financing. Both may recognize they can't get the deal done any other way.
So, you have to be crystal clear with the parties what was done and how they negotiated. When I brokered residential, I did a good amount of owner-financing. Sometimes it affected the sale price. Sometimes it didn't. Usually, it was the only way to get a property sold.